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This CL moves the method -setActive: to the coordinator layer since
it deals with many child coordinators.

Change-Id: I8e61ece1380169ffc10e9e00bccaa68dbde7d4d4
Reviewed-on: https://chromium-review.googlesource.com/c/1355924Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613219}